Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications.
By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality. NaNi0.5Mn0.5O2 is a known Na-ion material in which the nickel is present as Ni2+ while the manganese is present as Mn4+. The material is ordered with the Na and Ni atoms residing in discrete sites within the structure. The nickel ions (Ni2+) are a redox element which contributes to the reversible specific capacity and the manganese ions (Mn4+) play the role of a structure stabilizer. Compound NaNi0.5Ti0.5O2 is analogous to NaNi0.5Mn0.5O2 in that the Ni2+ ions provide the active redox centre and the Ti4+ ions are present for structure stabilization. There is plenty of literature describing the preparation of NaNi0.5Mn0.5O2 (and to a lesser extent NaNi0.5Ti0.5O2) as the precursor for making LiNi0.5Mn0.5O2 and LiNi0.5Ti0.5O2 by Na→Li ion exchange for Li-ion applications. A direct synthesis method to make these Li materials yields undesirable disordered materials, for example, as a result of the lithium and nickel atoms sharing structural sites. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi0.5Mn0.5O2 electrodes in propylene carbonate electrolyte solutions. The results obtained show that although NaNi0.5Mn0.5O2 exhibits some reversible charging and discharging ability, the capacity of the material fades by 25% or more, after only 40 cycles.
It is typically possible to predict the maximum charge capacity for sodium and lithium nickelate compounds based on the Ni2+ to Ni4+ redox process, however as Zhonghua Lu and J. R. Dahn, J. Electrochemical Society, 149 (7) A815-A822 (2002) explain, the electrochemical behaviour of cells made using lithium containing compounds, e.g. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2, where x=⅙, ¼, ⅓ and 5/12 do not always follow this conventional wisdom. These workers report that when cells containing lithium-nickelate materials are charged at voltages up to 4.45V, this causes lithium to be removed until the Mn oxidation state reaches 4+; thus giving an expected charge capacity of 2×. However, when lithium cells where x<½ are charged to higher voltages, e.g. between 4.5 and 4.7V, they exhibit a long plateau approximately corresponding to 1-2× and subsequent to this plateau, these materials reversibly cycle at capacities over 225 mAh/g. Put simply, lithium-containing compounds of the formula Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 where the amount of nickel is less than 0.5, exhibit a significantly higher charge capacity than would be expected from conventional theoretical calculation. By contrast, Lu and Dahn note that a similar increase in charge capacity is not observed when x=0.5, i.e. LiNi0.5Mn0.5O2, as there is sufficient nickel present to remove all of the lithium. To explain the higher than expected charge capacity phenomenon, Lu and Dahn demonstrated that their lithium materials undergo a non-reversible loss of oxygen when they are charged to higher voltages, and these oxygen deficient materials then react reversibly with lithium. Notwithstanding this, although this increased charge capacity is a potentially interesting discovery, the commercial utility of such compounds is hindered by the high cost of lithium, as discussed above.
US2007/0218361 teaches a sodium ion secondary battery which comprises a positive electrode active material which includes a sodium-containing transition metal oxide NaaLibMxO2±α. The M includes at least two of manganese, iron, cobalt and nickel. In such materials it is apparently extremely important that the amount of sodium is neither too large (otherwise excess sodium oxide or sodium hydride is produced which causes the positive electrode to become highly moisture absorptive) nor too small. In the latter case the amount of sodium ions that can be intercalated and de-intercalated is said to be reduced and this results in a high discharge capacity not being able to be obtained. This prior art describes that the optimum amount of sodium, i.e. the value of a, is preferably 0.6 to 1.1, further preferably 0.9 to 1.1 and more preferably 1. Meanwhile the amount of oxygen is also described as critical to performance. Too much oxygen is said to occupy transition metal and/or alkaline metal sites, and presumably this will hinder re-intercalation of the sodium ions during charge/discharge. Too little oxygen is said to produce material with a crystal structure with lots of defects. The optimum range for a is from 0 to 0.1. Another feature of the specific examples described in US2007/0218361 is that they all contain manganese in oxidation state +3. This is to allow sodium extraction by a manganese oxidation process (Mn+3→Mn4+).
The present invention aims to provide a cost effective electrode that contains an active material that is capable of achieving a considerably higher specific charge capacity than would be expected from conventional theoretical calculations. Further, it is desirable for such active materials to be straightforward to manufacture and easy to handle and store. Further still, the present invention aims to provide an electrode which is able to be recharged multiple times without significant loss in charge capacity. In particular the present invention will provide an energy storage device that utilises an electrode of the present invention for use in a sodium-ion cell or a sodium metal cell.
Therefore in a first aspect, the present invention provides an electrode containing one or more active materials comprising:AaM1VM2WM3XM4yM5ZO2−c  (Formula 1)                wherein        A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;        M1 is nickel in oxidation state less than or equal to 4+,        M2 comprises a metal in oxidation state less than or equal to 4+,        M3 comprises a metal in oxidation state 2+,        M4 comprises a metal in oxidation state less than or equal to 4+, and        M5 comprises a metal in oxidation state 3+        wherein        0≦a≦1, preferably 0≦a<0.5        v>0, preferably 0<v<0.5        at least one of w and y is >0        x≧0, preferably x>0        z≧0        c>0.1, preferably 0.1<c≦0.5where (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.        
Preferred electrodes of the present invention contain one or more active materials comprising:AaM1VM2WM3XM4yM5ZO2−c  (Formula 1)                wherein        A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;        M1 is nickel in oxidation state 4+,        M2 comprises a metal in oxidation state 4+,        M3 comprises a metal in oxidation state 2+,        M4 comprises a metal in oxidation state 4+, and        M5 comprises a metal in oxidation state 3+        wherein        0≦a<1, preferably 0≦a<0.5        v>0, preferably 0<v<0.5        at least one of w and y is >0        x≧0, preferably x>0        z≧0        c>0.1, preferably 0.1<c≦0.5        where (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.        
Particularly preferred electrodes contain one or more active materials comprising:AaM1VM2WM3XM4yM5ZO2−c  (Formula 1)                wherein        A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;        M1 is nickel in oxidation state less than 4+,        M2 comprises a metal in oxidation state less than or equal to 4+,        M3 comprises a metal in oxidation state 2+,        M4 comprises a metal in oxidation state less than or equal to 4+, and        M5 comprises a metal in oxidation state 3+        wherein        0≦a≦1, preferably 0≦a<0.5        v>0, preferably 0<v<0.5        at least one of w and y is >0        x≧0, preferably x>0        z≧0        c>0.1, preferably 0.1<c≦0.5        where (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.        Metals M2 and M4 may be the same or a different metal in oxidation state less than or equal to 4+. Moreover M2 and M4 are interchangeable with each other. When M2=M4 then Formula 1 may be written either as:AaM1VM2WM3XM4YM5ZO2−c,orAaM1VM2W+YM3XM5ZO2−c,orAaM1VM3XM4Y+WM5ZO2−c,        and all of these forms of the equation are to be regarded as equivalent.        
Compounds having sodium alone as the chosen alkali metal are especially preferred.
Preferably, M2 comprises a metal selected from one or more of manganese, titanium and zirconium; M3 comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt; M4 comprises a metal selected from one or more of manganese, titanium and zirconium; and M5 comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.
As discussed below, the active materials of Formula 1 are surprisingly capable of providing a specific charge capacity that is considerably higher than that predicted from conventional theoretical calculations.
Especially preferred electrodes contain one or more active materials comprising:AaM1VM2WM3XM4yM5ZO2−c  (Formula 1)wherein    0≦a≦1; preferably 0≦a<0.5; further preferably 0≦a≦0.4; and highly preferably 0.1≦a≦0.4;    v>0, preferably 0<v<0.5;    0<w≦0.5;    0≦x<0.5; preferably 0<x<0.5;    0<y<0.5;    z≧0; and    c≧0.11; preferably c≧0.13 and highly preferably c≧0.15, preferably the upper limit for c is ≦0.5,where a, v, w, x, y, z and c are chosen to maintain electroneutrality.
In a highly preferred active material of the present invention, (v+w)<0.8, especially when M2=Mn and/or Ti.
Advantageous electrodes of the present invention contain one or more active materials comprising:                Ni0.25Ti0.25Mg0.25Mn0.25O1.75;        Ni0.33Mn0.33Mg0.167Ti0.167O1.83;        Ni0.33Mn0.33Cu0.167Ti0.167O1.833;        Ni0.33Mn0.33Zn0.167Ti0.167O1.833;        Ni0.33Mn0.33Ca0.167Ti0.167O1.833;        Ni0.4.5Mn0.45Mg0.05Ti0.05O1.95;        Ni0.35Mn0.35Mg0.15Ti0.15O1.85;        Ni0.3Mn0.253Mg0.2Ti0.2O1.8;        Ni0.4Mn0.254Mg0.251Ti0.251O1.9;        Ni0.4Mn0.4Mg0.0.5Ti0.05Al0.1O1.9;        Ni0.35Mn0.35Mg0.05Ti0.05Al0.2O1.85;        Ni0.33Mn0.33Mg0.11Ti0.11Al0.11O1.83;        Ni0.3Mn0.3Mg0.05Ti0.05Al0.3O1.8;        Ni0.35Mn0.35Mg0.1Ti0.1Al0.1O1.85;        Ni0.3Mn0.3Mg0.1Ti0.1Al0.2O1.8;        Ni0.33Mn0.33Al0.33O1.83;        Ni0.4Mg0.1Mn0.5O1.9;        Ni0.35Mg0.15Mn0.5O1.85;        Ni0.333Mg0.167Mn0.5O1.833;        Ni0.3Mg0.2Mn0.5O1.8;        Ni0.4Mg0.1Mn0.5O1.9;        Ni0.35Mg0.15Mn0.5O1.85;        Ni0.3Mg0.2Mn0.5O1.8;        Na0.2Ni0.25Ti0.25Mg0.25Mn0.25O1.85;        Na0.2Ni0.333Mn0.333Mg0.167Ti0.167O1.933;        Na0.2Ni0.333Mn0.333Zn0.167Ti0.167O1.933;        Na0.2Ni0.333Mn0.333Zn0.167Ti0.167O1.933;        Na0.2Ni0.333Mn0.333Ca0.167Ti0.167O1.933;        Na0.05Ni0.45Mn0.45Mg0.05Ti0.0.5O1.975;        Na0.2Ni0.35Mn0.35Mg0.15Ti0.15O1.95;        Na0.2Ni0.3Mn0.3Mg0.2Ti0.2O1.9;        Na0.1 Ni0.4Mn0.4Mg0.1Ti0.1O1.95;        Na0.1 Ni0.4Mn0.254Mg0.05Ti0.05Al0.1O1.95;        Na0.2Ni0.35Mn0.35Mg0.05Ti0.05Al0.2O1.95;        Na0.2Ni0.333Mn0.333Mg0.111Ti0.111Al0.11O1.933;        Na0.2Ni0.3Mn0.3Mg0.0.5Ti0.05Al0.3O1.9;        Na0.2Ni0.35Mn0.35Mg0.1Ti0.1Al0.1O1.95;        Na0.2Ni0.3Mn0.3Mg0.1Ti0.1 Al0.2O1.9;        Na0.2Ni0.333Mn0.333Al0.333O1.933;        Na0.1Ni0.4Mg0.1Mn0.5O1.95;        Na0.2Ni0.35Mg0.15Mn0.5O1.95;        Na0.2Ni0.333Mg0.167Mn0.5O1.933;        Na0.2Ni0.3Mg0.2Mn0.5O1.9;        Na0.2Ni0.3Mg0.2Mn0.5O1.9;        Na0.2Ni0.35Mg0.15Mn0.5O1.95; and        Na0.2Ni0.3Mg0.2Mn0.5O1.9.with Ni0.33Mn0.33M30.167Ti0.167O1.833 and Na0.2Ni0.33Mn0.33M30.167Ti0.167O1.933 being the most favourable materials.        
The active materials used in the electrodes of the present invention may be prepared using any known method. However, a particularly convenient method involves effecting the net loss of Na2O from one or more active cathode materials of Formula 2, where Formula 2 is defined asA′a′M1′V′M2′W′M3′X′M4′y′M5′z′O2  (Formula 2)                wherein        A′ comprises either sodium or a mixed alkali metal in which sodium is the major constituent;        M1′ is nickel in oxidation state 2+,        M2′ comprises a metal in oxidation state 4+,        M3′ comprises a metal in oxidation state 2+,        M4′ comprises a metal in oxidation state 4+, and        M5′ comprises a metal in oxidation state 3+        1≦a′<2;        0<v′<0.5;        0<w′≦0.5;        0≦x′<0.5;        0≦y′<0.5;        z′≧0;        and wherein v′, w′, x′, y′ and z′ are all chosen to maintain electroneutrality.        
A highly efficient process of producing compositions of Formula 1 comprises:                charging an electrochemical cell containing one or more active cathode materials of Formula 2 beyond the conventional theoretical specific capacity as determined by the Ni2+/Ni4+ redox couple; wherein Formula 2 is defined as:A′a′M1′V′M2′W′M3′X′M4′y′M5′z′O2  (Formula 2)        wherein        A′ comprises either sodium or a mixed alkali metal in which sodium is the major constituent;        M1′ is nickel in oxidation state 2+,        M2′ comprises a metal in oxidation state 4+,        M3′ comprises a metal in oxidation state 2+,        M4′ comprises a metal in oxidation state 4+, and        M5′ comprises a metal in oxidation state 3+        1≦a′<2;        0<v′<0.5;        0<w′≦0.5;        0≦x′<0.5;        0≦y′<0.5;        z′≧0;        and wherein v′, w′, x′, y′ and z′ are all chosen to maintain electroneutrality.        
The compounds of Formula 2 are disclosed in Applicant's patent applications GB1212263.6, GB1212268.5 and GB1212261.0 which documents are incorporated herein by reference.
The process of overcharging involves charging the electrochemical cell containing the one or more active cathode materials to a specific capacity greater than 180 mAh/g. Typically this means charging the Na-ion cell to at least 4.2V. Preferably the overcharging process is the first charge performed on the active materials although it may be possible to charge the active materials to their “normal” specific capacity first and follow this with a process in which the materials are overcharged.
Charging doped sodium nickelate compounds of Formula 2 up to their theoretical capacity causes the oxidation of Ni2+ to Ni4+ and the removal of a portion of the sodium ions from the lattice. By contrast, charging such compounds above their conventional theoretical capacity, i.e. “over-charging” them, produces materials of Formula 1 in which                M1 is nickel in oxidation state 4+,        M2 comprises a metal in oxidation state 4+,        M4 comprises a metal in oxidation state 4+,        0≦a<1, preferably 0≦a<0.5 and        C>0.1, preferably 0.1<c≦0.5.        
The enhanced charge capacity (or anomalous charge capacity) observed when materials of Formula 2 are “over-charged” is due to several changes that occur as the over-charging is taking place. Broadly speaking this is a combination of the conventional (or “normal”) charge process using just the oxidation of Ni2+ to Ni4+, together with other structural changes that occur exclusively at the higher voltages, such as the removal of a higher than expected (from conventional theory) number of sodium ions and also an irreversible partial loss of oxygen to yield O2-c-containing active materials.
The present invention therefore provides a method of increasing the specific charge capacity of an oxide-containing cathode composition for use in a Na-ion cell comprising:                charging the oxide-containing cathode composition in an electrochemical cell to cause the loss of oxygen from the oxide-containing cathode composition and thereby form an oxygen deficient oxide-containing cathode composition.        
Further the present invention provides an oxygen deficient oxide-containing cathode composition, for example made using the above method.
Advantageously the present invention provides an electrode comprising an oxygen deficient oxide-containing cathode composition, for example made using the above method.
The mechanism by which oxygen is lost from the active cathode composition during the overcharging process operates most efficiently when the oxide-containing active cathode composition comprises a layered structure. Layered AxMO2 materials are known to adopt several, very similar structural types, the most well-known of which have been categorised by Delmas et al. Physica B+C, 91, (1980), 81, as “03, P2 and P3”. These notations describe the stacking of the layers; the letter refers to the way in which the oxygen ions are coordinated to the alkali ions, with the letter 0 denoting an octahedral coordination and the letter P denoting a prismatic coordination. The number refers to the number of MO2 layers contained in the unit cell. In the case of the uncharged active cathode materials used in the electrodes of present invention, it is preferred that they adopt a layered α-NaFeO2 type structure (space group R-3m, no. 166), which has an 03 crystal structure. Following charging, these materials are thought to adopt 03, P2 or P3 or modified versions of these crystal structures, although it is also possible that other crystal structures may be adopted.
The Applicant has noted that when layered active cathode materials, for example of Formula 2, are over-charged in a Na-ion cell, they undergo a structural change, which produces a reduction in the volume of the unit cell as compared with both the pristine precursor material, e.g. materials of Formula 2 before charging, and the precursor material after it has been charged to its conventional theoretical capacity, e.g. based on Ni2+ to Ni4+. It is believed that such unit cell volume reduction is consistent with the loss of alkali metal-ions and oxygen loss. This is discussed further below in the relation to the Specific Examples. Another characteristic of overcharged oxide-containing cathode compositions is that they appear to lose crystallinity (compared with to the uncharged material); this is shown by the FWHM of the 003 peak of the XRD pattern.
A determination of when a material is said to be “overcharged” is made by observing the voltage curve for the particular material. At capacities less than or close to the conventional theoretical maximum charge capacity (i.e. the Ni2+/Ni4+ redox couple) a first voltage feature is seen. However the curve continues to rise to a second voltage feature upon further or “over” charging. The materials are said to be “over-charged” from the beginning of this second voltage feature.
Therefore the present invention provides a method of optimising the specific charge capacity of an oxide-containing cathode composition for use in a sodium-ion cell comprising charging the composition onto at least a portion of a voltage feature which is observed as a result of the oxide-containing cathode composition being charged above the conventional theoretical maximum charge capacity.
The present invention also provides a method of optimising the specific charge capacity of an oxide-containing cathode composition in a sodium-ion cell comprising:                a) charging the sodium-ion cell beyond the conventional theoretical capacity of the cathode based on the Ni2+/Ni4+ redox couple; and        b) degassing the Na-ion cell to remove gasses formed during the charging process.        
A doped sodium nickelate of Formula 2 charged to its theoretical specific capacity limit based on the Ni2+/Ni4+ redox couple, will re-yield a compound with no oxygen loss when discharged; i.e. at this level, charging is seen to be reversible. However, discharging compounds of Formula 2 that have been “over-charged” results in producing materials of Formula 1 in which                M1 is nickel in oxidation state less than 4+,        at least one of M2 and M4 comprises a metal in oxidation state less than 4+,        0<a≦1        0<v<0.5 and        c>0.1, preferably 0.1<c<0.5.        
Consequently, during the discharge of over-charged compounds of Formula 2 the nickel is reduced from oxidation state 4+ to oxidation state 2+, some or all of the sodium-ions are re-inserted into the lattice and the lost oxygen atoms are not replaced. Moreover, although the metals M2 and/or M4 do not contribute anything to the initial overcharging process, upon discharge of the overcharged material it is found that at least one of the metals M2 and/or M4 is also reduced to an oxidation state less than 4+. This allows some or all of the sodium ions that were removed during the first “over-charge” to be reinserted into the material, and therefore produces a material with a higher discharge capacity. It is clear from this description that the role of the Mn4+ (or Ti4+) in the uncharged materials of the present invention is to allow for the reduction (Na insertion) process during cell discharge. This is in direct contrast to the role of the manganese in US2007/0218361 which is to allow Na extraction by the Mn oxidation process Mn3+→Mn4+ during the first charge process.
The role of the M3 and M5 metals is to reduce the amount of nickel in the doped nickelate materials of Formula 1, thereby allowing the “over charge” mechanism to take place as oxygen is released from the structure. The presence of the M3 and M5 metals also reduces the relative formula mass (formula weight) of these materials, thereby further improving the cost effectiveness of the electrode materials. In order to achieve anomalous capacity, there must be over twice the amount of sodium as there is nickel in the pristine materials (e.g. Formula 2) so that, when all Ni2+ has oxidised to Ni4+, there is still some Na left in the material, which can then be removed when the material is overcharged and releases oxygen. This explains why anomalous capacity is not observed for NaNi0.5Mn0.5O2; in this case there is enough Ni2+ present to remove all of the Na. In each of the compounds of Formula 2 where a′=1, following the requirement above, there has to be less than 0.5 Ni and the overall material must be electroneutral. Satisfaction of these two conditions is facilitated by the presence of an M3 metal, and/or an M5 metal.
When oxide-containing cathode compositions have undergone the over-charge/discharge process described, it is found that they may be cycled in the Na-ion cell between normal voltage limits for the Ni2+/Ni4+ redox couple, typically this might be expected to be between 1 and 4V, and no further unconventional voltage plateaux are formed. Moreover, cycling the over-charged/discharged compositions at normal voltage limits maintains a higher specific capacity than would be expected from conventional theoretical calculations, i.e. the over-charged/discharged oxide-containing cathode compositions perform better than the pristine compounds when cycled using the same normal voltage limits.
Thus the present invention provides a method of using an oxide-containing cathode composition according to Formula 1 in a Na-ion cell comprising:                a) charging a Na-ion cell comprising an oxide-containing cathode composition according to Formula 2 to a cell voltage beyond the conventional theoretical capacity of the cathode material based on the Ni2+/Ni4+ redox couple;        b) degassing the Na-ion cell to remove gasses formed during the charging process; and        c) cycling the resulting Na-ion cell, which now comprises an oxide-containing cathode composition according to Formula 1, over a voltage range within the normal voltage limits of the Ni2+/Ni4+ redox couple.        
As mentioned above, it is envisaged that the optimized oxide-containing cathode composition of the present invention may be prepared by a method that does not rely on overcharging a compound of Formula 2. Thus the present invention includes an optimised oxide-containing cathode composition made by any route.
The electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.
Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).
The present invention also provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as a sodium-ion cell and/or a cell in which sodium is the major mobile ion; a cell in which sodium is the major alkali metal-ion; either of which may be used in a non-aqueous or an aqueous electrolyte system.
It is also possible to convert sodium-ion derivatives into mixed lithium-ion/sodium-ion materials using an ion exchange process.
Typical ways to achieve Na to Li-ion exchange include:    1. Mixing the sodium-ion material with a lithium-containing material e.g. LiNO3, heating to above the melting point of LiNO3 (264° C.), cooling and then washing to remove the excess LiNO3 and side-reaction product    2. Treating the Na-ion material with an aqueous solution of lithium salts, for example 1M LiCl in water; and    3. Treating the Na-ion material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.